1. Field of the Invention
The present invention relates to the production of derivatives of the antimalarial artemisinin. More specifically, the present invention relates to the production of artemisinin derivatives via microbial hydroxylation under conditions that preserve the peroxide bridge moiety necessary for antimalarial activity. Hydroxylated derivatives of artemisinin can be employed to produce useful alternatives to artemisinin itself.
2. Description of Related Art
Formidable problems encountered in developing a malaria vaccine (1) and the ability of Plasmodium falciparum to become resistant to new drugs (2) have stimulated interest in using combinations of drugs for treating this disease, which claims more than a million lives a year (3).
The occurrence of a number of drug resistant strains of malaria makes it unlikely that a single drug will be effective in treating both resistant and non-resistant parasites. Recent reports by Van Dyke et al. (2) describing the use of compounds, e.g., verapamil, tetrandrine, etc., to reverse the chloroquine-resistance in Falciparum malaria promise to rejuvenate efforts to find new combinations of drugs to treat malaria. Van Dyke has also demonstrated that these compounds act in a synergistic manner to greatly reduce the quantities of drugs required for activity. Verapamil and tetrandrine apparently act by interfering with a mechanism in malarial parasites that rapidly expels drugs and foreign compounds from the cell's interior. Similar mechanisms apparently provide multidrug resistance in cancer cells (13). As pharmacologists and others develop a better understanding of these mechanisms, they will be able to formulate strategies to circumvent these mechanisms and treat these diseases with drugs. Such short term solutions will permit others to complete the research efforts required to formulate ways of treating malaria and similar diseases with vaccines.
The finding by Van Dyke showing that there are mechanisms (pumps) in malaria parasites to remove foreign substances (drugs) before they act, and the existence of molecules capable of stopping this "pumping" action, should stimulate efforts to identify combinations of fast and slow acting drugs that can effectively be used to treat patients with malaria.
One drug of current interest for which P. falciparum has not developed resistance is artemisinin, (I), (qinghaosu) (4). First isolated by Chinese investigators (5) from a traditional medicinal herb, Artemisia annua L., it has also been isolated from Artemisia species in the U.S. (6). Qinghaosu has been shown to act rapidly and to be relatively non-toxic.
A report by Chinese investigators that the active component of an herb used to treat malaria is a sesquiterpene, artemisinin, aroused enormous interest in employing derivatives of this rapidly acting compound, which is structurally different from quinine and other antimalarials (2). In order to prepare derivatives with enhanced solubilities in water and in lipids, it was first necessary to reduce the lactone to a ketal that possess a hydroxyl group. Carbonates, esters, and ethers of dihydroartemisinin were prepared and tested. Results of those tests demonstrated that the peroxide bridge was absolutely essential for activity (5).
Investigators from the "Coordinating Clinical Study Group on Qinghaosu" prepared the first derivatives of dihydroartemisinin in China and employed them to treat 1511 cases of vivax malaria and 588 cases of falciparum malaria between 1973 and 1978 (12). The compounds uniformly yielded clinical cures. The most potent derivatives found by these investigators were carbonates. The second most active group of dihydroartemisinin derivatives was esters, followed by ethers. A major shortcoming of this work was the high recrudescence rate. In order to overcome this problem, new derivatives of dihydroartemisinin were sought.
Lee et al (10a) and (10b) have shown that microbial oxidation of artemisinin and arteether by a number of fungi yields products in which the peroxide bridge is replaced by an oxide. Such products are neither active against malaria, nor can they be converted into antimalarial drugs.